1. Field of the Invention
This invention relates to a reference electrode and, more particularly, to a reference electrode used in measuring ion concentration, gas concentration and the like. Further, the invention relates to a reference electrode capable of operating stably for an extended period of time in a biological system or circulating circuit system.
2. Description of the Prior Art
Examples of reference electrodes (also referred to as comparison electrodes) known in the art include saturated calomel electrodes and silver/silver chloride electrodes. These reference electrodes are readily available on the market and comprise a glass tube accommodating a saturated potassium or sodium chloride solution and an electrode. Formed in the distal end portion of the tube is a liquid-junction portion through which the solution of potassium or sodium chloride is allowed to flow out. When a measurement is to be taken in a living body or body fluid, use of the saturated calomel electrode is hazardous since the electrode relies upon mercury. In such cases, therefore, the silver/silver chloride electrode is employed However, the outflow of the potassium or sodium chloride solution in the latter electrode has a great effect upon a living body. For this reason, the liquid-junction portion is formed of a porous material to reduce the amount of outflow. Nevertheless, fully satisfactory results are not obtained.
Another disadvantage of the conventional reference electrode is that the electrode is used in a living body or in a circuit system through which a body fluid circulates, the potential of the electrode is rendered unstable by changes in temperature. Though a potential which remains stable for a long period of time can be obtained by adding a large quantity of potassium or sodium chloride crystals to the internal liquid chamber of the electrode or adopting a porous body as the liquid-junction portion, these expedients make it difficult to miniaturize the electrode.
Another type of reference electrode is adapted to enable replenishment of the sodium chloride which has flowed out. Such an electrode enjoys a comparatively long service life. However, in order to allow this reference electrode to operate stably for an extended period of time in a biological system or circulating circuit and to be integrated with any of a variety of sensors such as an ion sensor, the electrode is required to be of the solid-state type, small in size and possessed of a long life. However, a solid-state electrode of this kind does not enable the electrolyte to be replenished or replaced, and an expedient must be devised that reduces the amount of electrolyte outflow.